The phrase you need to research is Single Sign On. This is not simple if you need to be robust and serve lots of public users and your domains have different names. In a closed environment it gets easier as you don't have to take account of all the variables, you know which browsers yous support etc.
You can use a WSFederation, an OAuth provider, such as Google or Live, or you can implement a shared authentication service that your sites both access. But, there are restrictions: your specific requirement of using two separate domains will make it very difficult for you. You could also look at Claims based identity which may change your approach and make this easier for you.
###Simpler single sign-on (shared domain or sub domain)
I will look at simpler cases here as well, although if your question is taken literally you may not be able to do this. However you can see what your options are, and maybe you can change your domain names (or at least use subdomains). For sites:
- on the same domain in different subfolders(www.domain1.com/site1 and www.domain1.com/site2) this is easy, and this approach will work perfectly.
- in subdomains of the same domain (site1.domain1.com and site2.domain1.com) it is a little harder, but you can still make it work.
The basic approach is:
Set the machine key for authentication to be the same in both web.config files
If using different subdomains: Modify your FormsAuthentication cookie to be used for both site1.domain1.com and site2.domain1.com once you log someone in
// after successful login in your login action (and in any other place, such as Register):
HttpCookie authCookie = FormsAuthentication.GetAuthCookie(userName,
authCookie.Domain = "example.com"
Response.AppendCookie(authCookie);
Use a shared authentication database if you let a user login at either site instead of just the main one
Note, though, that the shared cookie for authentication is becoming increasingly hard due to browser restrictions, and this is written about in Global Network Auto-Login by Jeff Atwood himself. A full consideraton of the possible approaches is written about on meta in "How does SO's new auto-login feature work?". Again, therefore, if you have only to support a closed environment (such as your office, or a small set of customers with a known set of browsers) your life will be easier.
Shared authentication tokens and query strings
This is also possible if you are feeling brave, and if you have a shared database or service you can set up.
When a user logs in successfully on site1.com, you can setup a one-time token for that login in the database and put the details of that token into the cookie. When they get passed to site 2 you could pass that one-time token in your query string, and then within site 2 you can check to see if the token they have passed is valid. If it is then you can log them in and set their login cookie. You need to be careful with this, there are security considerations for passing what is essentially an identity within a query string.
Footnote
There is a great online MS book on claims based authentication called "A Guide To Claims-Based Identity And Access Control" (beware: pdf link), I have added it as a footnote in case of link rot.
